[Ground-station] Baseband => decimation - questions

Ahmet Inan xdsopl at gmail.com
Sun Jan 27 14:12:10 EST 2019


I actually have never built a direct conversion receiver but since reading
Dan's paper of his "Tayloe detector", which doesn't even need a Sine/Cosine
and Analog Mixer pair, I got intrigued by his concept and wanted to know
more about it.
So these are my findings about direct conversion, never tried but it makes
sense to me:

Do we have to worry about other signals in the vicinity of the frequency of
interest?
If no, then the roll-off of the analog filters should be enough and I would
also go with Ron's suggestion of direct conversion.
Then besides equalization, only one thing remains, that can be taken care
of in the digital domain:
Dealing with the unmatched input amplitudes and phase shifts, caused by the
diverging signal paths and component tolerances.

So one has to decide either to use a higher (at least twice the) sampling
rate for the IF to feed the FFT (or DEMOD) with one (real) signal to avoid
phase ambiguity or do the Zero IF direct conversion to get the analytic
signal pair with the problems described above.

And as always: Please tell me if I got something wrong :)

Ahmet

On Sun, Jan 27, 2019 at 10:08 AM Ron Economos via Ground-Station
<ground-station at lists.openresearch.institute> wrote:

> SDR's like the Ettus B2x0, bladeRF and LimeSDR are direct conversion.
> Here's a block diagram of the architecture.
>
> [image: direct conversion]
>
> The PLL is tuned to the frequency of interest. The IF is 0 Hz. Each ADC
> runs at the desired sample rate and provides 1/2 the sample rate of
> bandwidth. The analog baseband low-pass filters are set to 1/2 the
> bandwidth. No decimation required.
>
> In fact, the bladeRF has no filtering at all in it's FPGA. If you set the
> baseband analog filters to wider than the desired bandwidth, you can see
> the aliasing (the TX is also direct conversion). The sample rate for this
> OFDM signal was 6.86 MHz.
> [image: aliasing]
>
> Same signal with the baseband filter set properly to 5.5 MHz (each
> low-pass filter set to 2.75 MHz).
>
> [image: no alias]
>
> Ron W6RZ
> On 1/26/19 23:51, Zach Leffke via Ground-Station wrote:
>
> I'll attempt to clarify without adding to much noise......I think I'm
> seeing both sides of this here......Though I'm no DSP expert, so fair
> warning I might misspeak a bit.
>
> A lot of what has been discussed so far is about down conversion, not
> decimation.........
>
> The trick is the 'guts' of the SDR, specifically the RFIC and the ADC
> used.  Say we have an IF of 700 MHz coming out of an LNB.  The upper edge
> of our signal would be at 705 MHz.  According to Nyquist, you would have to
> sample that IF signal at 1410 Megasamples per second (at least).  Any lower
> than that, and images of the sampled signal will overlap and corrupt the
> data (Aliasing).  Say your ADC had 14 bits.......that's a lot of data,
> probably more than most FPGAs can handle.
>
> The trick in the SDR (pick your vendor, I'm staying generic here) is that
> it has a tunable front end that performs another level of down conversion
> in the analog domain before sampling (for example, the AD9361 in Ettus
> B210s, E310s, etc).  This takes the 700 MHz signal down to say 20 MHz (made
> that number up), with an ADC running at a fixed rate..... say 80 MHz (also
> made that number up).
>
> At this point, my DSP-fu is weak, but where I'm going with this is that
> the 80 MHz sampling is real samples, represents 40 MHz of RF bandwidth
> (Nyquist).  This can be represented as a stream of complex samples (IQ)
> running at half the rate, though each sample is twice the size, one for I
> and one for Q .......(Nyquist isn't violated, because even at half the
> rate, you have two samples, one In-phase, and one Quadrature)......
>
> So now we have a stream at 40 MSPS (complex), spanning from 0 to +40 MHz,
> with our signal camped out at +20 MHz.  So then we tune digitally to center
> our signal at 0 MHz...aka complex baseband.  Now we have a stream still at
> 40 MSPS (complex), but spanning from -20 MHz to +20 MHz.  Negative
> frequencies are OK in the complex domain.  Our signal of interest is now
> centered at 0 MHz, but is spanning from -5 MHz to +5 MHz.
>
> I think what Michelle is getting at is that we don't want to have an FPGA
> processing a 40 MHz wide stream of data, when we only need to worry about
> 10 MHz.  Since our signal is centered at 0 MHz, we can start throwing away
> samples........Decimation.  We can toss 3 out of every 4 samples out
> (decimate by 4, and filter), which leaves us with a 10 MSPS stream of
> complex samples.  This also has some benefit in that we are also rejecting
> the noise contributions of those unnecessary samples that we tossed out
> (linked to the B in kTB).
>
> Everything that was (horribly) described above is handled 'under the hood'
> by the UHD drivers in Ettus products for example.  When you tell a UHD
> source block the sample rate you want (usually the 'samp_rate' variable
> that shows up in the flowgraph) what you are actually telling it is the
> 'requested' sample rate on the output of the above process.  The onboard
> ADC still runs at the fixed higher rate (80 Mhz in my example), but based
> on your input into the UHD source block, it will automatically select the
> right parameters to ensure that after sampling, conversion to complex
> baseband (including tuning your requested center frequency to 0 MHz),
> decimation and filtering, that the rate you requested is fed out to the
> host computer.  This is why you have to be careful about selecting sample
> rates with UHD.........some of you have probably seen the debug output
> where it warns you that the division of the ADC clock rate by the requested
> sample rate is not an even integer and to expect 'CIC
> rolloff'........basically it couldn't cleanly do the decimation you
> requested (again....something here about the value of half band filters in
> the FPGA that are part of the conversion to complex baseband and
> decimation.....sorry for my weak DSP-fu).  You know you got it wrong when
> you see a 'hump' of spectrum when the spectrum should be 'flat'.
>
> Now we can run that 10 MSPS complex stream of samples through the
> demodulation process and extract the 'frames of interest' for any
> particular user........that would be demultiplexing (throw out the frames
> for everyone else, I only want the frames for me).
>
> I would offer that on the ground side, there is no channelizer in the
> mix.....you must receive the entire 10 MHz signal to recover the full
> downlink data stream, since there is only one time multiplexed, 10 MHz wide
> signal.
>
> On the satellite payload on the other hand....you WILL want a
> channelizer.  Lets say the uplink is 10 MHz wide, and supports 1000
> channels, so each 10 kHz wide.  In order to demodulate 1000 channels in
> parallel, somewhere in there you need to tune 1000 times to center each
> uplink signal to complex baseband and decimate to 10 ksps (complex).
> executing 1000 'tunes' in parallel at the full 10 MSPS rate is very very
> wasteful........Think of the channelizer as a really efficient way of
> performing the tuning and decimation so that each output channel of the
> channelizer is only 10 ksps and it is properly 'centered' on the desired
> uplink channel.  it will require more horsepower than a single tune and
> single decimate 'string' but is less processing intensive than 1000 of
> those strings running in parallel.
>
>
> I didn't mention anything about other sampling tricks (Nyquist zones, and
> potential spectral inversion issues), or the benefits of oversampling
> (dynamic range), integer vs float representation.........maybe on another
> thread one day.
>
> So if you are going to roll your own hardware..........first a lot of
> downconversion from 10 GHz (maybe an LNB to get to say L-Band or maybe
> something to get to a 'ham band IF' at 432 or 144).  Thats not enough.
> You'll have to then downconvert again to something that can be handled by
> the selected ADC and whatever clock rate it is running at (Nyquist rules
> apply here).  Filtering and careful consideration of mixing products will
> matter!  phase noise of the LOs will matter in all that
> downconversion.......Also, gain gain gain... will matter to make sure that
> you are fully exercising the full range of the ADC and not just toggling
> the the lowest couple of bits  (but not too much gain.....clipping).  Then
> the complex baseband conversion and decimation and on to demodulation,
> demultiplexing, etc. etc.......A lot of the above is what is handled 'under
> the hood' by most of the commercial SDRs out there (i.e. UHD) so that the
> end user can easily get up and running with the 'more interesting' stuff
> downstream.........
>
> Hopefully this helped clarify the issue.....sorry if it added more noise
> (my DSP-fu is weak).  Not sure if I actually answered any questions.......
>
>
> -Zach, KJ4QLP
>
> Research Associate
> Aerospace Systems Lab
> Ted & Karyn Hume Center for National Security & Technology
> Virginia Polytechnic Institute & State University
> Work Phone: 540-231-4174
> Cell Phone: 540-808-6305
>
> On 1/25/2019 7:27 PM, Ron Economos via Ground-Station wrote:
>
> Okay. De-multiplexing is a much better and less confusing terminology. As
> you stated, decimation is a DSP thing and channelizing the downlink payload
> has nothing to do with DSP (all the DSP has already been down in order to
> deliver payload packets).
>
> Ron W6RZ
> On 1/25/19 16:17, Michelle Thompson wrote:
>
> To me, decimation is what we do in order to channelize in the payload.
>
> I don't think that's exactly what I'm being asked about in the ground
> station receiver, though.
>
> -Michelle W5NYV
>
>
>
>
> On Fri, Jan 25, 2019 at 4:14 PM Ron Economos <w6rz at comcast.net> wrote:
>
>> I'm not sure we are talking about the same thing yet. So what exactly do
>> you expect to decimate and why?
>>
>> Ron W6RZ
>> On 1/25/19 16:07, Michelle Thompson wrote:
>>
>> The beginning of wisdom being the definition of terms and all, it would
>> be good to make sure we're all talking about the same thing.
>>
>> So far, I've used LNBs and USRPs for receive, with the LNB doing an IF at
>> 618MHz (LNB-on-a-Stick) and giving reasonable performance.
>>
>> Decimation to me is a DSP thing, or used to reduce power consumption when
>> you don't need to sample as high as you can.
>>
>> -Michelle W5NYV
>>
>>
>>
>>
>> On Fri, Jan 25, 2019 at 3:52 PM Ron Economos via Ground-Station
>> <ground-station at lists.openresearch.institute>
>> <ground-station at lists.openresearch.institute> wrote:
>>
>>> The standard IF for DVB-S2 receivers is 950 to 2150 MHz.
>>>
>>> DB6NT was selling a down-converter from 10489-10500 MHz to 1129-1140 MHz
>>> for P4A.
>>>
>>>
>>> https://shop.kuhne-electronic.com/kuhne/en/shop/new/MKU+LNC+10+OSCAR+P4A/?card=1832
>>>
>>> I'm not sure what decimation has to do with receiving DVB-S2. The entire
>>> 10 MHz signal needs to be demodulated. Individual baseband frames will be
>>> selected for processing, but I call that de-multiplexing.
>>>
>>> Ron W6RZ
>>> On 1/25/19 15:32, David Vieira via Ground-Station wrote:
>>>
>>> Michelle - Thanks for posting.  I'll frame some of the questions.
>>>
>>> Typical 10 GHz terrestrial contesting rigs are Heterodyne; that is a
>>> Mixer works with a Local Oscillator (LO) to take the RF down to an IF
>>> (Intermediate Frequency).
>>> For an SDR, that IF can be digitized by an Analog-Digital Converter.
>>>
>>> The most popular IF for contesting/SSB rigs is 144 MHz.
>>> For a data BW of 10 MHz that may or may not be a fast enough IF
>>> carrier.  If we can digitize and recover the data, it would allow a lot of
>>> re-use of existing equipment.
>>>
>>> I've heard suggestions/proposals up to the 1.2 GHz Ham band.
>>> In some sense, the IF carrier could be 144/220/440/915/1200 MHz, or even
>>> any Non-Ham frequency in between.
>>>
>>> There are a lot of proof of existence designs for a 10 GHz Mixed down to
>>> an IF; and lots of off the shelf ADC dev-boards.  (catch me off thread for
>>> details).
>>>
>>> Some questions I have are:
>>> ---from an FPGA side of the SDR, what data rate(s) can the FPGA absorb
>>> in to a decimator?
>>>
>>> Must we decide upfront on a single frequency; or
>>> preferably allow flexibility in the RF front end design (ie, Mixer, PLL
>>> and Local Oscl hardware choices) by allowing a wide and programmable
>>> variety of ADC and decimation rates?
>>>
>>> {This is where RF and Digital folks must communicate across walls.}  ;-)
>>>
>>> Comments welcome.
>>>
>>> regards,
>>> David
>>> KI6CLA
>>>
>>>
>>> On Friday, January 25, 2019, 2:41:54 PM PST, Michelle Thompson via
>>> Ground-Station <ground-station at lists.openresearch.institute>
>>> <ground-station at lists.openresearch.institute> wrote:
>>>
>>>
>>> While we are striving to enable all sorts of wonderful designs by
>>> putting prototypes into GNU Radio, a central goal is to design our own
>>> hardware.
>>>
>>> We've had a lot of progress on the protocol and algorithm front (GSE,
>>> LDPC, some of the polyphase).
>>>
>>> Some fundamental decisions about our own hardware need to be made.
>>>
>>> When we receive, we expect to have to decimate. This is because we are
>>> receiving at a relatively high frequency (10GHz).
>>>
>>> Our bandwidth is (up to) 10MHz. For DVB-S2/X, we fix our sampling rate,
>>> depending on what bandwidth we want to support. We have a lot of freedom
>>> here.
>>>
>>> Picking the right frequencies for the receive chain is therefore
>>> important.
>>>
>>> What are our options?
>>>
>>> What options make the best sense?
>>>
>>> I'd like to build and test as soon as possible, so let's get some
>>> discussion going.
>>>
>>> -Michelle W5NYV
>>>
>>>
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>>
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-- 

Ahmet Inan

Co-founder and CEO of aicodix GmbH
https://www.aicodix.de/
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